Ryazhenka (also spelled Ryazhanka) is a variation on yogurt of Ukrainian origin, in which the milk is slowly cooked before it is cultured.    The process causes Maillard reactions, which gives the ryazhenka a tannish color, and thickens the milk through evaporation.  Traditionally, it would be baked at a low temperature in a traditional oven, but it is easier in an American kitchen to do the cooking step in a crock pot.  The photo at the top shows ryazhenka on the left (tan colored and thicker) and yogurt made without the slow cooking step on the right. 

It takes about 1-1/2 days to make ryazhenka. It was noticeably less tart than the traditional yogurt.  Carol suggested I should use it as an ingredient in yogurt cheeses, such as Suzma .

2 quarts whole milk

¼ cup yogurt with active culture

Put the milk in a crock pot with a lid on the high setting until the milk reaches the 180-200 degree range.  Reduce the heat setting and put the lid a bit ajar so that the evaporation can escape.  Cook the milk for 8 hours periodically adjusting the heat setting to keep the milk temperature in the target zone.  On my crock pot this was easy to do.

Here is what the milk looked like after 8 hours of cooking.

Whisk the crust into the milk while the milk is cooling down to 110 degrees.  Then whisk in the yogurt with active culture and culture the yogurt in your normal way.  Here is how I make Yogurt , which is in one-quart jars using my sous-vide equipment.